Pygmy three-toed sloth

The pygmy three-toed sloth (Bradypus pygmaeus), also known as a monk sloth or dwarf sloth,[3] is a small three-toed sloth, endemic to Isla Escudo de Veraguas, a small island off the coast of Panama, which separated from the mainland nearly 8900 years ago.[4] Only described as a separate species in 2001, they are thought to have originated from isolation of individuals of the mainland population of brown-throated three-toed sloths. The population became a distinct species through insular dwarfism on the island.

Studies suggest an inverse, linear relationship between mean body sizes and age of the island for island populations of sloths in this region.[5]

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Pygmy three-toed sloths have a tan face with a dark brown band across the brow and orange eye patches. The back can exhibit either uniform or blotchy color distribution, but is usually dark brown with an obvious dorsal stripe. Pygmy sloths are unique in that they have long hairs on the crown and the sides of the head, giving the distinct impression of a hood.[5] Compared to the related brown-throated three-toed sloth, the pygmy species is, on average 40% smaller in body mass, weighing 2.5 to 3.5 kilograms (5.5 to 7.7 lb), and 15% smaller in body length. Adults measure 48 to 53 centimetres (19 to 21 in), with a 4.5 to 6.0 centimetres (1.8 to 2.4 in) tail.

They have a relatively small skull, with a large external auditory meatus, narrow squamosal and mandibular processes, no foramina in the anterodorsal nasopharynx, a minuscule stylomastoid foramen, and they usually lack foramina for the external carotid artery.[4] They have eighteen teeth, ten in the upper jaw and eight in the lower. Two of the teeth in each jaw are incisor-like, although those in the upper jaw are small or may be absent. The incisor-like teeth in the lower jaw are compressed anteroposteriorly. Many of the features found in pygmy sloths are thought to be indicative of a relatively rapid evolution of a new species in an isolated, island, habitat.[4]

Pygmy sloths are also 12–16% smaller than in cranial dimensions than the mainland species (length: 67.5 to 72.2 millimetres (2.66 to 2.84 in); width: 38.8 to 45.7 millimetres (1.53 to 1.80 in).[4]

All three-toed sloths are arboreal mammals that feed on leaves; the pygmy sloth is unique in that it is found exclusively in the red mangroves, and feeds on coarse leaves. Red mangrove leaves are a relatively poor source of nutrients, in comparison with the tender leaves of the Cecropia tree eaten by brown-throated sloths on the mainland.[5]

The smaller size of pygmy sloths reduces their energy requirements for survival and reproduction, making them an apparent example of insular dwarfism.[5] No predators of pygmy three-toed sloths have been documented.

Mating, gestation, birth and post-birth dynamics have not been observed for pygmy sloths, but these features may be inferred from studies of other species in the genus[citation needed].

Individuals of other species reach sexual maturity around three years of age and typically give birth after twelve months gestation,[5] although captive bred sloths can give birth as early as six months after mating.[6] Mammary glands are found near the armpits of the female and infants cling to the mothers’ underside.[7] Captive-bred young of other species are independent of their mothers around six months of age.[6] Some reports suggest that female sloths give birth to a single offspring, but observations of a female brown-throated sloth in the wild with two infants suggest that they are capable of producing twins.[6]

The behavior of pygmy three-toed sloths has not been reported, but can be inferred from the behavior of its close relative, the brown-throated sloth.[citation needed]

Two male brown-throated sloths were observed fighting in the wild by striking one another using their forefeet. This observed dispute probably took place over access to new greenery and fruits in a Cecropia tree. In other cases, disputes between male sloths may be for rights to mate. Captive females will fight for resources.[8] Like other sloths, the pygmy sloth is a good swimmer.[citation needed][9]

A 2011 study found only 79 pygmy three-toed sloths on Escudo de Veraguas.[10] While their population has presumably always been low due to their restricted range, this census found far lower population numbers than had been estimated (around 300).[11] Although the island has no human population, visiting fishermen poach the sloth, which is an easy target because it only lives in the mangrove forests by the sea. Although protected as a wildlife refuge, the enforcement is lax.[11]